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On The Generation Of Animals   

Everything then exists for a final cause, and all those things which
are included in the definition of each animal, or which either are
means to an end or are ends in themselves, come into being both
through this cause and the rest. But when we come to those things
which come into being without falling under the heads just
mentioned, their course must be sought in the movement or process of
coming into being, on the view that the differences which mark them
arise in the actual formation of the animal. An eye, for instance, the
animal must have of necessity (for the fundamental idea of the animal
is of such a kind), but it will have an eye of a particular kind of
necessity in another sense, not the sense mentioned just above,
because it is its nature to act or be acted on in this or that way.

These distinctions being drawn let us speak of what comes next in
order. As soon then as the offspring of all animals are born,
especially those born imperfect, they are in the habit of sleeping,
because they continue sleeping also within the mother when they
first acquire sensation. But there is a difficulty about the
earliest period of development, whether the state of wakefulness
exists in animals first, or that of sleep. Since they plainly wake
up more as they grow older, it is reasonable to suppose that the
opposite state, that of sleep, exists in the first stages of
development. Moreover the change from not being to being must pass
through the intermediate condition, and sleep seems to be in its
nature such a condition, being as it were a boundary between living
and not living, and the sleeper being neither altogether
non-existent nor yet existent. For life most of all appertains to
wakefulness, on account of sensation. But on the other hand, if it
is necessary that the animal should have sensation and if it is then
first an animal when it has acquired sensation, we ought to consider
the original condition to be not sleep but only something resembling
sleep, such a condition as we find also in plants, for indeed at
this time animals do actually live the life of a plant. But it is
impossible that plants should sleep, for there is no sleep which
cannot be broken, and the condition in plants which is analogous to
sleep cannot be broken.

It is necessary then for the embryo animal to sleep most of the time
because the growth takes place in the upper part of the body, which is
consequently heavier (and we have stated elsewhere that such is the
cause of sleep). But nevertheless they are found to wake even in
the womb (this is clear in dissections and in the ovipara), and then
they immediately fall into a sleep again. This is why after birth also
they spend most of their time in sleep.

When awake infants do not laugh, but while asleep they both laugh
and cry. For animals have sensations even while asleep, not only
what are called dreams but also others besides dreams, as those
persons who arise while sleeping and do many things without
dreaming. For there are some who get up while sleeping and walk
about seeing just like those who are awake; these have perception of
what is happening, and though they are not awake, yet this
perception is not like a dream. So infants presumably have
sense-perception and live in their sleep owing to previous habit,
being as it were without knowledge of the waking state. As time goes
on and their growth is transferred to the lower part of the body, they
now wake up more and spend most of their time in that condition.
Children continue asleep at first more than other animals, for they
are born in a more imperfect condition than other animals that are
produced in anything like a perfect state, and their growth has
taken place more in the upper part of the body.

The eyes of all children are bluish immediately after birth; later
on they change to the colour which is to be theirs permanently. But in

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